Why married people are so different then single people?

Since I m born in November I was a year ahead in school. My friends have always been older because of that. When I moved to Canada, I skipped grade 9 completely putting even more separation between people my age and me.

Mainly because of where I went to high school a lot of people didn't go on to university, those that did and I continued way we did before; you know study, party, travel with little worry in the world. Those who didn't started getting married and having things before I finished my second year of university. Ok, weird but to each its own.

After 4 years, of what I still consider pretty awesome part of my life, I graduated and was expected to go and get a job and live my life. If you remember reading this, I took a little break.  I decided to focus on my career; and before I knew, my Facebook was blowing up with what seems was my entire graduating class getting married and having kids. At this point, I felt a little bit left out...

When my close friends (of nearly ten years) got married and had kids we slowly fell apart. I was amazed how much people change after marriage and/or having kids. I felt we had nothing to talk about; I don't know anything about kids & frankly not interested. I cant remember what show it was, when they make fun of "we people" but thats what I felt. Here I was, giving it all I got at work, traveling and surrounded with "we people". Closest friends of mine would bring their husbands for coffee/dinner/movies or whatever it was. At one point if my girlfriend would say she is coming I would have to always count plus one. Not that I don't like her spouse, I do, but you can't talk you used to when you have a "stranger" just sitting there. Year or so later, we had completely separate lives where neither fit into the others.

Is it crazy that one simple event like that can alter a friendship completely? What do you think?